1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to arrangements for detecting the derailment of the counterweight in a traction elevator system, such as caused by an earthquake.
2. Description of the Prior Art
Damaging earthquakes are likely to occur in certain well defined earthquake zones of the world. The need for earthquake detection, or damage detection, in certain elevator systems installed in earthquake zones was recognized after the California earthquake in 1971.
U.S. Pat. No. 3,791,490 discloses an arrangement for detecting abnormal horizontal counterweight movement, with the arrangement including a vertically oriented wire strung tautly in the hoistway. A metallic ring mounted directly to the counterweight frame encircles the wire without contact therewith during normal operation. Horizontal movement of the counterweight such that the ring contacts the wire, grounds the wire through the counterweight frame, wire supporting ropes, drive sheave and bearings. When grounding of the wire is detected, the resulting signal is used to modify the operation of the elevator system. For example, the elevator car may be stopped with regard to floor level, and the car may then be operated at a reduced speed to the closest floor in a direction away from the counterweight.
With certain buildings and building heights it is possible for the wire to sway and/or vibrate to such an extent that it contacts the ring on the counterweight during normal operation of the elevator system. When this happens, the elevator car is stopped and taken out of service unnecessarily.